1. The Technical Field
The present invention relates to gas heat exchangers.
2. The Prior Art
There are many applications in which it is desirable to use gas heat exchangers. These include applications where it is desired to cool down a gas, for example in exhaust gas recirculation (EGR) coolers, and applications in which the heat extracted from the gas can be recovered, for example to warm the cabin of a vehicle.
Under some circumstances, heat exchange may be required, but under other circumstances it may be undesirable. For example, at the beginning of operation of a vehicle, heat may be extracted from exhaust gas to assist in warming up the cabin of a vehicle. In this case, heat extraction is required. However, once the cabin reaches the required temperature, further warming could be undesirable. Therefore under these circumstances, heat extraction is not required.
Another example is exhaust gas recirculation. Exhaust gas recirculation is a method of reducing noxious emissions from internal combustion engines. In particular, the presence of exhaust gas in the combustion mixture reduces the tendency to form NOx compounds.
In general it is advantageous to cool the re-circulated exhaust gas. Its reduced temperature helps to lower the combustion temperature within the cylinder of the engine. EGR gas is also more dense when cooled, and therefore for a given mass of gas, a lower volume of air is displaced from the combustion chamber. Cooling the EGR gas is not desirable under all conditions however. When engine temperature is low and under low engine loads it is often preferable to recirculate EGR gas without cooling.
Many applications requiring heat exchange, including EGR systems, therefore require a bypass, to control whether the gas is cooled or not. When it is required for heat losses from the gas to be reduced, the gas is diverted through the bypass channel. When it is required for heat losses from the gas to be increased, that is to say, when the gas is being cooled, the efficiency of the cooler should be high, and the gas is not passed through a bypass channel.
An exhaust gas recirculation cooler typically comprises at least one gas cooling conduit configured to carry gas, at least one conduit configured to carry a coolant fluid, and a bypass conduit. The coolant conduit and the gas cooling conduit are in close proximity, such that gas that is transported through the gas cooling conduit is in proximity to coolant fluid and therefore is cooled down. When gas cooling is required, then the gas is diverted to be carried by the gas cooling conduit. Under circumstances where gas cooling is not required, then the gas is diverted through the bypass conduit. A bypass valve controls whether the gas is carried in the gas cooling conduit or the bypass conduit. For EGR applications, the bypass valve is separate from an EGR valve, which controls whether EGR gas is flowing at all.
When gas is being transported through the bypass conduit, it is undesirable for the gas to be cooled. To achieve this, there should be little or no contact between the bypass conduit and the coolant conduit, as coolant fluid in the coolant conduit would cool gas that is transported through the bypass conduit under bypass conditions. Prior art solutions to minimise contact between the bypass conduit and the coolant conduit are already known.
It is known to use an external bypass channel, for example US2003150434, and WO03085252. The external bypass channel takes up additional space, which is a disadvantage for applications where packaging in the engine space is restricted. However, this solution is used because the bypass conduit is external to the exhaust gas cooling conduit and the coolant conduit and therefore the bypass conduit is not cooled by the coolant conduit.
A solution to this has been provided in U.S. Pat. No. 6,718,956 in which the bypass conduit is disposed within the main housing. The housing comprises a coolant conduit, in which a series of gas cooling conduits are disposed and also in which a bypass conduit is disposed. The bypass conduit is therefore in contact with the coolant in this type of cooler, which is undesirable. Complicated modifications are required to minimise the degree of cooling between the coolant fluid and the bypass conduit when the exhaust gas is flowing through the bypass conduit. These include having a double-walled bypass conduit with a vacuum between the two walls to reduce a heat exchange between the coolant fluid contained in the coolant conduit and the exhaust gas carried by the bypass conduit.
Another disadvantage of a bypass conduit is that the material in the bypass conduit acts as a heat sink when hot EGR gas is diverted through it. A transient period occurs during which some heat is extracted from the gas by contact with the conduit wall.
An alternative to having a bypass conduit is to have a valve that controls whether the coolant fluid flows through the coolant conduit or not. In this instance, a bypass conduit for the gas is not required. If cooling of the exhaust gas is required, then coolant flows through the coolant conduit and the exhaust gas is cooled. If cooling of the exhaust gas is not required then coolant does not flow through the coolant conduit and therefore the exhaust gas is not cooled. However, there are problems with this type of system. For example, when cooling is not required, residual coolant can be left in the coolant conduit. Coolant fluid typically contains volatile additives, which may be given off when the conduit is heated. The formation of steam and volatile substances is not desirable.